Spermatogonial stem cells from testes of adult mice and germline stem cells from testes of neonatal mice can generate pluripotent cells with developmental potential similar to embryonic stem (ES) cells. These results suggest that germ cells from the testes could be used to provide stem cells for tissue engineering and regenerative medicine. Hypotheses that ES cells may be derived from germ cells provide a similar impetus for understanding the mechanisms that specify and maintain the germ cell lineage. Current understanding of germ cell specification mechanisms is based largely upon studies of model organisms that utilize cytoplasmic determinants to specify their germ cells (e.g., Drosophila and C. elegans). In contrast, many organisms (including mammals) utilize inductive signals to specify germ cell fate. Studies of inductive specification have been limited almost exclusively to mouse and many questions remain unanswered. For example, are there conserved inductive signals that specify germ cell fate? What are the mechanisms that activate germ cell-specific patterns of gene expression? The freshwater planarian, Schmidtea mediterranea, serves as a useful model for studying these questions. It has prodigious regenerative abilities, based upon a population of stem cells, that allow it to regenerate a complete animal from a tiny body fragment;the germ cell lineage can also be regenerated. This work will capitalize on the functional genomic tools available for studying S. mediterranea to dissect the mechanisms by which inductive signals specify germ cell fate. Microarray analyses will be performed to identify genes that are expressed differentially between animals that have early germ cells and animals that lack them. Next generation sequencing technologies will be utilized to characterize gene expression changes during the initial course of germ cell development. High through put in situ hybridization will be used to validate candidate genes identified by these analyses. The functions of these differentially expressed genes will then be examined by RNA interference. These studies have the potential to identify conserved genes that are required for proper germ cell development.